An electronic device configured as an environmental sensor may include a sensor integrated circuit (IC) housed within the cavity of a cavity-type package. To enable the sensor IC to sense particular characteristics of the environment, the package may include an opening (e.g., in the package lid), which permits exposure of the sensor IC to the external environment. In some types of sensors, such as pressure sensors, a moisture-permeable material (e.g., a gel) may be disposed inside the cavity and over the sensor IC, in order to protect the sensor IC from otherwise potentially destructive exposure to the external environment. In such a device, the quantity of gel disposed within the cavity is selected so that an air gap is present between the surface of the gel and the package lid. During a sensing operation, the force of air pressure impinging upon the exposed surface of the gel is translated through the gel to the pressure sensor IC. The pressure sensor IC acts as a transducer, generating an electrical signal indicative of the pressure imposed.
In some cases, moisture entering the package cavity may result in reliability and/or accuracy issues for such sensor devices. For example, moisture may diffuse through the relatively-permeable, protective gel and lodge on the surface of the package cavity on which the sensor IC is mounted. Under certain conditions (e.g., at elevated temperatures, such as during a reflow process), the moisture may transition to a gaseous state, producing bubbles at the interface between the gel and the package cavity surface. Those bubbles may thereafter migrate to the exposed surface of the gel. When the bubbles are sufficiently large and/or prolific, they may significantly deform the surface of the gel so that the gel contacts the package lid. The bubbles also may migrate to the surface of the sensor IC. Either of these conditions, in turn, may produce a mechanical offset and, thus, inaccuracy in the sensor measurements that may be obtained by the device. In extreme cases, bubble-caused gel surface deformation may substantially eliminate the air gap, and gel may ooze through the package opening, thus compromising the functionality of the sensor device.